A GPS is a fantastic tool. It makes your flights a lot easier (and safer) and a lot less stressful because you know 100% sure where you are at any given time...no second guessing, no mental overload.
I got my GPS shortly after I became uncomfortable about my position (maybe two years after getting my PPL).
The the essence of your question seems to be about whether you should get it before or after your CPL. Having never done a CPL, I'm not really in a good position to advise you on that, so will leave it to the other.
But one thing that I did want to comment on is the idea being posted by some of having it as a backup that you can call on if things start to go wrong. I do not think that it's a good idea having a GPS in the flight bag to bring out to save the day when things go wrong. The reason for this is because GPS's rely on their last known position to help speed up the fix. They also use a database that is updated regularly to help predict where the satelites are going to be.
If your GPS hasn't been used in some time then both of these pieces of information will be well out of date, meaning that your GPS is trying to find it's own position based on out of date information. This means that on the ground with a good view of the sky, it may take some time. Put it inside the aircraft (without an external antenna such as would be the case with a portable unit) with an out of date almanec (database of where the satelites are) will put it at a significant disadvantage. Then take into account that the aeroplane is moving along quite fast (and hence your position constantly changing) and you are really making the GPS work to find its position.
With a modern chip such as the sirf III then it will take some time to get things sorted out...could easily be quite a number of minutes. With a Sirf II or earlier, then you couldn't even be 100% sure that it will ever find it's position until you have stopped moving.
So in an emergency, the GPS that has lived in your bag for the past couple of months without being used, is more likely to be a useless distraction than a 'get out of jail free card'.
I remember reading about someone on the Flyer forums a few years ago, who had their GPS for emergencies only, who was having difficulty finding a grass strip, and was running out of daylight. They pulled out the backup GPS expecting it to save the day, and it never managed to get a signal.
I tell you this as a Beta Tester of PocketFMS, so I should be encouraging you to buy a GPS system! But I would in all honesty advise you not to buy one if it's just going to live in your flight bag to save the day in an emergency.
GPS's are also complicated systms, and to use one correctly, you should have some regular practice with it long before the emergency, which again isn't really in tune with the 'in the bag for an emergency'.
The only way that I see it as being an 'in an emergency' system, but not used for nav normally, is if you set it up, get a signal, then put it somewhere out of sight...eg on the back side windows. Then it will have the signal when you need it, but isn't available while you are trying to practice your dead reckoning. But temptation to look at it will always be strong!
Incidently in this senario it does have a second advantge, or being able to look at your flight tracks post flight and see where you made any mistakes in your DR. PocketFMS, and some of the others too, will allow you to output your breadcrumbs to Google Earth. You can then see what caused problems in your DR. In my case for example, i was able to see in a number of flights, that I had a habit of turning over the start of a town, rather than over the actual centre, and I could then see how this magnified errors over the rest of the flight. So in this senario it does have some advantages.
But don't buy one to leave in your flight bag until it's needed.
dp